Axial flow fan with blades twisted according to a blade pitch ratio that decreases (quasi) linearly with the radial position

ABSTRACT

An axial flow fan has a hub, an annular shroud extending concentric with the axis of the hub, and a number of fan blades connected at a root end to the hub and having a free tip end extending radially towards the shroud. The blades are designed so that the fan in operation at nominal operating conditions generates a fluid flow in the immediate vicinity after the blades, which includes a core flow that is substantially a forced vortex flow where the tangential flow speed component (c 2,u ) is proportional with the radial distance (r) from the axis. Equations define the chord angle θ of the blades.

The present invention relates to an axial flow fan having an advantageous design of the rotor blades.

BACKGROUND

Axial flow fans are well known in the art and many different designs have been proposed and manufactured in order to improve the performance of the fan, in particular with respect to generation of noise and improved power efficiency. It is an object of the present invention to provide an improved fan design to improve the efficiency of the fan.

BRIEF DESCRIPTION OF THE PRESENT INVENTION

The present invention relates in a first aspect to an axial flow fan comprising a hub rotatable about an axis, an annular shroud extending concentric with said axis in a radial distance from said hub, a plurality of fan blades connected at a root end to said hub and having a free tip end extending radially towards said shroud, fan driver coupled to said hub and arranged for driving the rotation of said hub around the axis, wherein the angle Δθ between the chord of the blade at any radial position in the radial range from the hub and to the radial position of r/R=0.3 and the chord at the radial position of r/R=0.3 substantially follows the curve defined as

${\Delta \; \theta} = {{{arc}\; {\tan\left( \frac{{1.2\frac{R}{r}} - 0.2}{\pi} \right)}} - {50.4{^\circ}}}$

where R is the radial distance from the axis and to the free tip end of the blade and r is the radial distance from the axis and to the radial position. Such blade has a steeper chord angle θ at the region near the hub and tends to drive the core of the flow right after the rotor in a so-called forced vortex, which appear to improve the efficiency of the fan. With the term substantially is herein understood, that the angle Δθ deviates less that 4°, preferably less than 2° from the curve defined by the equation.

In a particularly embodiment, the angle Δθ between the chord of the blade in the radial range from the hub and to the radial position of r/R=0.4 and the chord at the radial position of r/R=0.4 substantially follows the curve defined as

${\Delta \; \theta} = {{{arc}\; {\tan\left( \frac{{1.2\frac{R}{r}} - 0.2}{\pi} \right)}} - {41.7{^\circ}}}$

With the term substantially is herein understood, that the angle Δθ deviates less that 4°, preferably less than 2° from the curve defined by the equation.

Furthermore, the pitch angle of the blades of the fan are in preferred embodiments of the first aspect of the present invention such that the angle θ between a plane of rotation of the blades and the chord of the blade at the radial position of r/R=0.3 is in the range of 40 to 60°, preferably in the range of 45 to 55°, such as substantially 50.4°.

According to a second aspect of the present invention, it relates to an axial flow fan comprising a hub rotatable about an axis, an annular shroud extending concentric with said axis in a radial distance from said hub, a plurality of fan blades connected at a root end to said hub and having a free tip end extending radially towards said shroud, fan driver coupled to said hub and arranged for driving the rotation of said hub around the axis, wherein the angle θ between a direction of rotation of the blades and the chord of the blade at all positions along the radial extent of the blade substantially follows the curve defined as

$\theta = {\arctan\left\lbrack {\frac{1}{15\; \pi^{2}}{\frac{Flow}{{nD}^{3}}\left\lbrack {\frac{1.46}{\frac{r}{R}} + {0.564 \cdot \frac{r}{R}} - 0.77} \right\rbrack}} \right\rbrack}$

where R is the radial distance from the axis and to the free tip end of the blade, r is the radial distance from the axis and to the radial position, Flow is the nominal flow for the fan given in [m³/h], D=2·R is the rotor diameter given in [m] and n is the nominal rotational speed of the fan given in [rpm]. The fan according to the second aspect of the present invention will, when operating near the nominal operational conditions drive the core of the flow right after the rotor in a so-called forced vortex, which appear to improve the efficiency of the fan. With the term substantially is herein understood, that the angle θ deviates less that 4°, preferably less than 2° from the curve defined by the equation.

According to a third aspect of the present invention, it relates to an axial flow fan comprising a hub rotatable about an axis, an annular shroud extending concentric with said axis in a radial distance from said hub, a plurality of fan blades connected at a root end to said hub and having a free tip end extending radially towards said shroud, fan driver coupled to said hub and arranged for driving the rotation of said hub around the axis, wherein the blades are designed so that the fan when in operation at nominal operating conditions generates a fluid flow in the immediate vicinity after the blades which comprises a core flow that substantially is a forced vortex flow where the tangential flow speed component is proportional with the radial distance from the axis. The existence of such core flow is readily detectable by means of standard fluid flow measurement techniques, such as hot wire anemometry, laser-doppler velocimetry or particle image velocimetry.

The core flow extends preferably to a radial distance of at least 0.2 times the radial distance from the axis to the tip of blades, preferably the core flow extends in the range of 0.2 to 0.3 times said radial distance.

Furthermore, the fluid flow in the immediate vicinity after the blades outside the core flow is preferably substantially a free vortex flow where the tangential flow speed component is inversely proportional with the radial distance from the axis to the radial position, whereas the axial flow speed component is substantially constant.

For the fans according to each of the three aspects above, the inner radius of the annular shroud is in the range of 600 to 1500 millimetres. Also, the radius of the hub is preferably in the range of 50 to 75 millimetres which surprisingly has shown to improve the efficiency of the fan considerably, probably due to the reduced disturbance of the wake of the hub.

A fan according to the present invention preferably comprises a diffuser arranged concentric with said axis at a downstream position of the annular shroud. In a preferred embodiment, the diffuser has a conical shape with a diffusion angle in the range of 2 to 15° to the axis of rotation of the rotor, preferably in the range of 6 to 10°.

Furthermore, a fan according to the present invention comprises an inlet part provided with a bellmouth arranged concentric with said axis at an upstream position of the annular shroud.

According to a particular aspect of the present invention, it relates to an axial flow fan comprising a hub rotatable about an axis, and at least one fan blade connected at a root end to said hub and having a free tip end extending radially away from said axis, the hub comprising a seating part allowing the blade or blades to be arranged in a plurality of blade pitch angles and blade locking part for locking the blade pitch angle of the at least one blade into a specific blade pitch angle, wherein said locking part is designed to lock said blade into one specific blade pitch angle only. In one known embodiment in the art, a loose locking pin is inserted into one of a plurality of openings in the blade root or in the seating part in order to lock the blade pitch in one of a plurality of possible pitch angles. However, at maintenance or repair of the fan rotor, such pin may easily be reinserted into a wrong opening resulting in an erroneous blade pitch of one or more of the blades of the fan, causing inferior performance with respect to efficiency, air flow and/or noise generation. With the present invention, such errors may be prevented efficiently. In a preferred embodiment, the fan comprises one locking part for each of the blades, and in a particularly preferred embodiment, the blades each comprises a recess cooperating with a corresponding pin of the locking part.

BRIEF DESCRIPTION OF THE DRAWING

Embodiments of the present invention are illustrated with the enclosed drawing of which

FIG. 1 is a longitudinal schematic cross-section of an axial flow fan,

FIG. 2a shows the tangential flow speed of a fan designed according to the invention,

FIG. 2b shows the axial flow speed of a fan designed according to the invention,

FIG. 3 shows a blade of an axial flow fan according to the invention,

FIG. 4 illustrates a cross-section of a blade together with the flow direction, the direction of blade movement and the angle θ between the blade chord and the direction of movement,

FIG. 5 is a perspective view of a hub part, three blades and three locking parts for an axial flow fan according to an aspect of the present invention, and

FIG. 6 shows three different designs of the locking parts of FIG. 5.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE PRESENT INVENTION

The axial flow fan 1 of the present invention may be equipped with any convenient number of blades 2. In the embodiment of FIG. 5, a fan 1 having three blades 2 is shown, but it is generally preferred that the rotor 5 of the fan comprises from 3 to 6 blades.

The axial flow fan 1 as illustrated in FIG. 1 comprises a number of blades 2 connected to a hub 3. The rotor 5, i.e. the hub 3 and the blades 2 is rotated about an axis 4 by means of a motor (not shown) arranged in the hub 3 or next to the hub 3 and connected thereto by means of a drive arrangement, e.g. a belt drive. The rotor 5 is arranged inside a cylindrical shroud 6 which is concentric arranged about the axis 4 to that there is a clearance between the tip 7 of the blades 2 and the shroud 6. The rotation of the rotor 5 drives a flow of air axially through the fan 1 in the direction of the arrow A in FIG. 1. The flow path through the fan 1 is illustrated by streamlines 8 indicated in FIG. 1. The shroud 6 is preceded by an inlet part 9 arranged upstream of the shroud, i.e. in the direction against the driven flow A, where the inlet part 9 also is concentric with the axis 4 and comprises a bellmouth to smoothen the flow at the inlet part 9 in order to avoid separation of the flow. The passage of the air flow through the rotor 5 causes a pressure increase which is further increased by regained part of the as kinetic energy present in of the air flow immediately after the rotor due to the axial velocity component by means of a diffuser 10 arranged downstream of the shroud 6 and concentric with the axis 4. The diffuser 10 has a conical shape with an diffusion angle of 8.5° to the centre line, i.e. to the axis 4 of rotation of the rotor 5. The blades 2 are attached by their root end 11 to the hub 3, preferably in a manner discussed later.

The air is accelerated in the inlet part 9 from rest far upstream of the fan 1 to the flow condition immediately before the rotor 5. Since no work is performed on the air or any heat exchange takes place, the total enthalpy of the air will remain constant, which for the typical operational conditions of an axial fan for ventilation purposes means that:

h _(1,t) =c _(p) ·t ₁+½c ₁ ² =h ₀+½0² =h ₀

where h_(1,h) is the total enthalpy of the air immediately before the rotor 5, c_(p) is the heat capacity of the air, t₁ is the temperature of the air immediately before the rotor 5, c₁ is the air speed immediately before the rotor 1 and h₀ is the total enthalpy of the air at rest far upstream of the fan 1.

Since the air in the embodiments is not interacting with inlet vanes or guides, the flow direction in the inlet part 9 will be considered to be purely axial, i.e. that the tangential component of the air speed c_(1,u)=0 and the axial component equals the air speed, c_(1,a)=c₁.

For the present invention, the design of the blades 2 of the fan 1 has the aim and task of forming a rotor 5 which is characterised in that it during operation of the fan 1 generates an air flow immediately after the air flow passage of the rotor 5, which is a combination of two so-called Beltrami flows, i.e. cylindrical flows with constant total enthalpy, see e.g. Michael H. Vavra, Aero-Thermodynamics and Flow in Turbomachines, John Wiley & Sons, Inc. 1960.

From a selected core radius r_(c) and to the tip 7 of the blade where r=R, the air flow is a free vortex flow where the tangential flow speed component c_(2,u) is inversely proportional with the radius: c_(2,u)=k₁/r, k₁ being a first constant, whereas the axial flow speed component is constant, c_(2,a)=const.≈c_(1,a). This is a fairly standard design principle of an axial flow fan. However, from the hub 3 to the core radius r_(c) the rotor 5 is designed to drive a forced vortex flow, where the tangential flow speed component c_(2,u) is proportional with the radius r: c_(2,u)=k₂·r, where k₂ is a second constant, whereas the axial flow speed component will increase somewhat towards the hub 3. Both of these flows automatically fulfil the radial equilibrium between the centrifugal forces and the radial pressure gradient. The inclusion of a forced vortex flow at the core of the flow field after the rotor 5 is a novel design principle of a axial flow fan and provides advantages over the known prior art.

The design principle according to the present invention causes the change in total enthalpy and thereby the work performed by the blades 2 of the rotor 5 on the air flow to be substantially constant over the whole cross-section of the rotor 5 which is advantageous for the efficiency of the rotor 5. By incorporating a forced vortex flow at the core of the flow field after the rotor 5, preferably with a core radius r_(c) in the range of 0.2 to 0.3 the maximum tangential speed in the wake of the rotor 5 is reduced as well as the force of the whirl formed after the hub 3 which have a disadvantageous effect on regaining kinetic energy in the downstream diffuser 10.

Based on this design principle, the blades of a fan may be designed by the use of standard fan design tools when design parameters as fan diameter, flow rate and rotational speed of the rotor. An example of the flow immediately after the passage of the rotor 5 for an axial flow fan 1 with core radius r_(c)=0.2 is shown in FIG. 2 where the tangential air flow speed is shown in FIG. 2a and the axial air flow speed is shown in FIG. 2 b.

It has been found that the angle θ between the blade chord 14 and the direction of movement M of the blade 2 of a blade 2 of a fan rotor 5 designed according to the above design principle for the inner part of the blade, i.e. from the hub 3 and at least to the radial position of r/R=0.3, preferably as far as r/R=0.4 can be approximated with the equation

$\begin{matrix} {\theta = {\arctan\left( \frac{{1.2 \cdot \frac{R}{r}} - 0.2}{\pi} \right)}} & \left( {{Eq}.\mspace{14mu} 1} \right) \end{matrix}$

A more sophisticated approximation is alternatively found with the following equation:

$\begin{matrix} {\theta = {\arctan\left\lbrack {\frac{1}{15\; \pi^{2}}{\frac{Flow}{{nD}^{3}}\left\lbrack {\frac{1.46}{\frac{r}{R}} + {0.564 \cdot \frac{r}{R}} - 0.77} \right\rbrack}} \right\rbrack}} & \left( {{Eq}.\mspace{14mu} 2} \right) \end{matrix}$

where Flow is the nominal flow for the fan given in [m³/h], D=2·R is the rotor diameter given in [m] and n is the nominal rotational speed of the fan given in [rpm].

This equation 2 is applicable to the whole blade as a good approximation but is less precise at the inner part of the blade 2 than equation 1.

A single blade 2 of the fan 1 is shown in FIG. 3 where the axis 4 of rotation of the rotor 5 is indicated, the distance R from the axis 4 and to the tip end 7 of the blade 2 and an indication of the radial position of r/R=0.3, i.e. where the distance r from the axis is 0.3 times the radial position R of the blade tip end 7. A cross section of a fan blade 2 is shown in FIG. 4 with indication of the leading edge 12 of the blade 2 as well as the trailing edge 13 and the chord line 14 extending there between, the length of the chord of the blade 2 being the distance between the leading edge 12 and the trailing edge 13. The blade 2 is moved in the direction indicated as M on FIG. 4 due to the rotation of the rotor 5 of the fan 1 during operation thereof, and the angle θ between the chord line 14 and the direction of movement M is indicated on the figure together with the direction A of the incoming air flow, the direction A being depicted as being perpendicular to the direction M of movement which is generally the case for an axial flow fan 1.

Three examples of blades 2 of a rotor 5 designed according to the above design principle are provided below at a number of relative radial positions r/R, comprising the chord length, the profile type and the chord angles 0 as determined by means of the design principle, and for comparison the chord angles as calculated by means of equation 1 and equation 2.

Example 1

In this example, the Flow is designed to be 45000 m³/h, the nominal rotational speed n is 425 rpm and the radius R of the rotor 5 is 1.5 m. The condition of the air is taken to be a temperature of 20° C., a pressure of 101300 Pa and a relative humidity of 80%.

r/R R θ Chord Profile θ Eq. (1) θ Eq. (2) [—] [m] [°] [m] [—] [°] [°] 0.114 0.080 72.9 0.140 NACA 0009 73.0 72.4 0.203 0.142 61.0 0.140 I 61.2 59.6 0.291 0.204 50.4 0.160 I 51.3 48.9 0.380 0.266 42.5 0.205 NACA 9406 43.3 40.6 0.469 0.328 34.9 0.190 I 36.9 34.2 0.557 0.390 29.3 0.175 I 31.9 29.4 0.646 0.452 25.6 0.160 I 27.8 25.8 0.734 0.514 23.0 0.145 I 24.5 23.0 0.823 0.576 21.2 0.130 I 21.8 20.9 0.911 0.638 20.0 0.115 I 19.6 19.3 1.000 0.700 19.6 0.100 NACA 6406 17.7 18.1

The first five columns of the table provide the parameters of the blade 2 as found by standard design tools, whereas the last two columns are the chord angles 0 found by means of equation 1 and equation 2, respectively.

Example 2

In this example, the Flow is designed to be 16000 m³/h, the nominal rotational speed n is 970 rpm and the radius R of the rotor 5 is 0.5 m. The condition of the air is taken to be a temperature of 20° C., a pressure of 101300 Pa and a relative humidity of 80%.

r/R R θ Chord Profile θ Eq. (1) θ Eq. (2) [—] [m] [°] [m] [—] [°] [°] 0.216 0.080 56.9 0.095 NACA 2408-TE 59.6 59.2 0.295 0.109 50.0 0.110 I 51.0 50.1 0.373 0.138 43.1 0.115 NACA 9406-TE 43.8 42.7 0.451 0.167 35.9 0.110 I 38.1 36.8 0.530 0.196 31.5 0.105 I 33.3 32.1 0.608 0.225 27.5 0.100 I 29.4 28.5 0.686 0.254 24.4 0.095 I 26.2 25.6 0.765 0.283 22.0 0.090 I 23.5 23.4 0.843 0.312 20.1 0.085 I 21.3 21.6 0.922 0.341 18.6 0.080 I 19.3 20.1 1.000 0.370 17.4 0.075 NACA 6406-TE 17.7 19.0

Example 3

In this example, the Flow is designed to be 22500 m³/h, the nominal rotational speed n is 950 rpm and the radius R of the rotor 5 is 0.5 m. The condition of the air is taken to be a temperature of 20° C., a pressure of 101300 Pa and a relative humidity of 80%.

r/R R θ Chord Profile θ Eq. (1) θ Eq. (2) [—] [m] [°] [m] [—] [°] [°] 0.216 0.080 56.9 0.095 NACA 2408-TE 59.6 59.2 0.295 0.109 50.0 0.110 I 51.0 50.1 0.373 0.138 43.1 0.115 NACA 9406-TE 43.8 42.7 0.451 0.167 35.9 0.110 I 38.1 36.8 0.530 0.196 31.5 0.105 I 33.3 32.1 0.608 0.225 27.5 0.100 I 29.4 28.5 0.686 0.254 24.4 0.095 I 26.2 25.6 0.765 0.283 22.0 0.090 I 23.5 23.4 0.843 0.312 20.1 0.085 I 21.3 21.6 0.922 0.341 18.6 0.080 I 19.3 20.1 1.000 0.370 17.4 0.075 NACA 6406-TE 17.7 19.0

Conclusions from the Examples

The three examples show that the deviation between designed chord angle θ and the one calculated with the use of Equation 1 is less than 1° at relative radial positions r/R>0.4. The deviation between designed chord angle θ and the one calculated with the use of Equation 2 is somewhat larger in the region near the blade root but is for the overall blade 2 a better approximation, i.e. within about 2°.

Extensive testing of the blades of fans designed according to the design principles has revealed that a change of pitch angle of the blade to adjust a fan designed for one nominal set of operational conditions to a different set of operational conditions, mainly a different flow rate through the fan and a different rotational speed n of the fan to a large extent preserve the advantages of the fan design, i.e. an improved overall efficiency of the fan as compared to traditionally designed fans, for which reason an axial fan having blades following the design principles generally have shown to exhibit the advantages, even though the blades are turned to another pitch and the fan is operated with other operational conditions than the original nominal set of operational conditions. These advantages have been apparent at pitch angles deviating at least about 10° from the designed pitch angle and are increasing for pitch angles deviating in the range of 5° from the designed pitch angle of the blade 2.

The assembly shown in FIG. 5 of a seating part 16 of the hub 3, three blades 2 and three locking parts 18 for an axial flow fan according to an aspect of the present invention. The blades 2 are at the root end 11 equipped with a projection 15 that allows the individual blade 2 to be seated in a blade seating opening 17 of the seating part 16 at any pitch angle of the blade 2 as desired, the projections 15 being rotatable in the U-shaped seating openings 17. The blade root protections 15 being equipped with a recess (not visible) designed for cooperating with a pin 20 having a rectangular cross-section, the pin 20 being extending from the body of a locking part 18 which is suited to the inserted into the blade seating opening 17 when the blade root projection 15 is in place so as to lock the pitch angle of the blade 2 to a specific blade pitch angle defined by the locking part 18. In FIG. 6 is shown three different locking parts 18 a, 18 b, 18 c where the pin 20 a, 20 b, 20 c are arranged at different positions to define different pitch angles of the blade 2. The locking parts 18 a, 18 b, 18 c are provided with side tracks 19 to accommodate the edges of the blade seating opening 17 of the seating part 16 of the hub 3.

By providing a fan 1 with such system of a seating part 16, blades 2 provided with a recess in the blade root projection 15 and locking parts 18 a, 18 b, 18 c defining one specific pitch angle of the blade 2, any possible erroneous re-assembly of the hub 3 after repair of the fan 1 resulting in erroneous blade pitch may be avoided.

LIST OF REFERENCE NUMERALS

-   1 Axial flow fan -   2 Blade -   3 Hub -   4 Axis of rotation of the rotor -   5 Rotor -   6 Shroud -   7 Blade tip -   8 Streamlines of air flow through fan -   9 Inlet part -   10 Diffuser -   11 Root end of blade -   12 Leading edge of blade -   13 Trailing edge of blade -   14 Chord line -   15 Blade root projection -   16 Seating part of hub -   17 Blade seating opening -   18, 18 a, 18 b, 18 c Locking part -   19 Side tracks -   20, 20 a, 20 b, 20 c Pin -   A Direction of incoming air flow before the rotor -   M Direction of movement of the blade -   r Distance from axis of rotation to a radial position of the rotor -   r_(c) Core radius -   R Distance from axis of rotation to tip end of blade -   R_(hub) Radius of the hub -   Flow Nominal flow for the fan given in [m³/h] -   D Rotor diameter D=2·R given in [m] -   n Nominal rotational speed of the fan given in [rpm] -   c_(p) Heat capacity of the air -   c₁ Air speed immediately before the rotor -   c_(1,a) Axial component of the air speed c₁ -   c_(1,u) Tangential component of the air speed c₁ -   c_(2,u) Tangential flow speed component immediately after rotor -   c_(2,a) Axial flow speed component immediately after rotor -   c_(2,a) _(_) _(tip) Axial flow speed component immediately after     rotor at tip end of blade -   h₀ Total enthalpy of the air at rest far upstream of the fan -   h_(1,h) Total enthalpy of the air immediately before the rotor -   k₁, k₂ First and second constants -   t₁ Temperature of air immediately before the rotor -   θ Angle between the blade chord and the direction of movement -   Δθ Angle between the chord of the blade and the blade chord at a     predetermined radial position, e.g. of r/R=0.3 or 0.4 

1. An axial flow fan comprising; a hub rotatable about an axis, an annular shroud extending concentric with said axis in a radial distance from said hub, a plurality of fan blades connected at a root end to said hub and having a free tip end extending radially towards said annular shroud, a fan driver coupled to said hub and arranged for driving a rotation of said hub around the axis, wherein an angle Δθ between a chord of the blade at any radial position in a radial range from the hub to a radial position of r/R=0.3 and a chord at the radial position of r/R=0.3 substantially follows a curve defined as ${\Delta \; \theta} = {{\arctan\left( \frac{{1.2\frac{R}{r}} - 0.2}{\pi} \right)} - {50.4{^\circ}}}$ where R is a radial distance from the axis to the free tip end of the blade, and r is a radial distance from the axis to the radial position.
 2. The fan according to claim 1, wherein the angle Δθ between the chord of the blade in the radial range from the hub to the radial position of r/R=0.4 and the chord at the radial position of r/R=0.4 substantially follows the curve defined as ${\Delta\theta} = {{{arc}\; {\tan\left( \frac{{1.2 \cdot \frac{R}{r}} - 0.2}{\pi} \right)}} - {41.7{^\circ}}}$
 3. The fan according to claim 1, wherein the angle θ between a plane of a rotation of the blades and the chord of the blade at the radial position of r/R=0.3 is in a range of 40 to
 60. 4. An axial flow fan 04 comprising; a hub rotatable about an axis, an annular shroud extending concentric with said axis in a radial distance from said hub, a plurality of fan blades connected at a root end to said hub and having a free tip end extending radially towards said shroud, a fan driver coupled to said hub and arranged for driving a rotation of said hub around the axis, wherein an angle θ between a direction (M) of a rotation of the blades and a chord of the blade at all positions along a radial extent of the blade substantially follows a curve defined as $\theta = {\arctan \left\lfloor {\frac{1}{15\; \pi^{2}}{\frac{Flow}{{nD}^{3}}\left\lbrack {\frac{1.46}{\frac{r}{R}} + {0.564 \cdot \frac{r}{R}} - 0.77} \right\rbrack}} \right\rfloor}$ where R is a radial distance from the axis to the free tip end of the blade, r is a radial distance from the axis to the radial position, Flow is a nominal flow for the fan given in [m³/h], D=2·R is a rotor diameter given in [m] and n is a nominal rotational speed of the fan given in [rpm].
 5. An axial flow fan comprising; a hub rotatable about an axis, an annular shroud extending concentric with said axis in a radial distance from said hub, a plurality of fan blades connected at a root end to said hub and having a free tip end extending radially towards said shroud, a fan driver coupled to said hub and arranged for driving a rotation of said hub around the axis, wherein the blades are designed so that the fan when in an operation at nominal operating conditions generates a fluid flow in an immediate vicinity after the blades which comprises a core flow that is substantially a forced vortex flow where a tangential flow speed component (c_(2,u)) is proportional with the radial distance (r) from the axis.
 6. The fan according to claim 5, wherein the core flow extends to a radial distance (r) of at least 0.2 times of the radial distance (R) from the axis to the free tip end of the blades.
 7. The fan according to claim 5, wherein the fluid flow in the immediate vicinity after the blades outside the core flow is substantially a free vortex flow where the tangential flow speed component (c_(2,u)) is inversely proportional with the radial distance (r) from the axis to the radial position, whereas an axial flow speed component (c_(2,a)) is substantially constant.
 8. (canceled)
 9. The fan according to claim 1, wherein an inner radius of the annular shroud is in a range of 600 to 1500 millimetres.
 10. The fan according to claim 1, wherein a radius (R_(hub)) of the hub is in a range of 50 to 75 millimetres.
 11. The fan according to claim 1, comprising a diffuser arranged concentric with said axis at a downstream position of the annular shroud.
 12. The fan according to claim 11, wherein the diffuser has a conical shape with a diffusion angle in a range of 2 to 15° to the axis of rotation of the rotor.
 13. The fan according to claim 1, comprising an inlet part provided with a bell-mouth arranged concentric with said axis at an upstream position of the annular shroud.
 14. An axial flow fan comprising; a hub rotatable about an axis, and at least one fan blade connected at a root end to said hub and having a free tip end extending radially away from said axis, the hub comprising a seating part-allowing the blade or blades to be arranged in a plurality of blade pitch angles and blade locking part for locking the blade pitch angle of the at least one blade into a specific blade pitch angle, wherein said locking part is designed to lock said blade into one specific blade pitch angle only.
 15. The fan according to claim 14, comprising one locking part for each of the blades.
 16. The fan according to claim 14, wherein the blades each comprises a recess cooperating with a corresponding pin of the locking part.
 17. (canceled)
 18. The fan according to claim 1, wherein the angle θ between a plane of a rotation of the blades and the chord of the blade at the radial position of r/R=0.3 is in a range of 45 to 55°.
 19. The fan according to claim 5, wherein the core flow extends in a range of 0.2 to 0.3 times of said radial distance (R).
 20. The fan according to claim 11, wherein the diffuser has a conical shape with a diffusion angle in a range of 6 to 10° to the axis. 